Eugene Stoner received a patent for a gas operated self-loading rifle which revolutionized firearm design and gave him a place in history next to the likes of Browning and Garand. The M16 variant is still a mainstay in many military arsenals. The AR15 as owned by many civilians is normally semi-auto only. The receiver is divided into the upper and lower groups. The trigger, hammer and associated mechanisms are located in the lower receiver. The hammer is powered by a double torsion spring with the hammer located between and supporting the coils with the spring's loop between the coils wrapped around the back of the hammer and with the spring legs normally acting against the trigger mechanism. The AR15 has a very simple layout for lower production cost, cleaning and maintenance. However, removing and especially installing the hammer pivot pin is hampered by the preload tension of the hammer spring. While attempting to install a hammer and spring assembly the spring legs are acting on the trigger section and urging the assembly both upward and rearward against the pressure being applied in an attempt to align the hammer pivot bore with a bore on each side of the lower receiver as the hammer pivot pin is passed through all three and its leading edge must overcome the retaining detent spring in the hammer itself. Even though in disassembly the pivot pin is normally displaced with a slightly smaller diameter pin tool when the pin tool is retracted the hammer and spring assembly can flex sideways upon retracting the pin tool from one side of the receiver and upon complete removal the pin tool the hammer and spring assembly can be propelled some distance by the preload spring force.